[dev.link] cmd/link, cmd/internal/goobj2: mmap object file in -newobj mode

[dev.link] cmd/link, cmd/internal/goobj2: mmap object file in -newobj mode

With the old object file format, we use mmap (if supported) to
read object files and back symbol data with mapped read-only
memory.

Do the same with the new object file format. This also
significantly reduces number of syscalls made to read object
files.

Currently we still do mmap in object file level, not archive
level. This is probably ok, as there shouldn't be many archives
that contain more than one object. If this is a problem we can
change that later.

Change-Id: Icae3ef14d8ed6adbee1b5b48d420e2af22fd9604
Reviewed-on: https://go-review.googlesource.com/c/go/+/197797
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>